Chemical Affinity in terms of Electromotive Force. 289 



Kohlrausch (Pogg. Ann. cxli. p. 456, and Ergdnz. vi. p. 35), 

 H. F. Weber (Phil. Mag. 1878, [5] v. p. 189), and J. Thorn- 

 sen (Wied. Ann. xi. p. 246, 1880), all of which valuations 

 lie between 1*088 and 1*132 volt when reduced to that unit 

 (and, in the case of Bosscha's results, corrected for an error of 

 about 8 per cent, in the value of the coil used by him). To 

 these may be further added the electrostatic valuations of 

 Sir W. Thomson and Latimer Clark, both of which lie near to 

 I'll volt. Favre (Comjrtes Renclus, lxix. p. 35) and Raoult 

 {Ann. Chem. Phys. [4] ii. p. 338, and iv. p. 392) obtained by 

 methods involving calorimetric measurements numbers repre- 

 senting the " galvanic heat " of a Daniell cell, and equivalent 

 to considerably smaller electromotive forces, their valuations 

 (23993 and 23900 gramme-degrees respectively) corresponding 

 to 1*058 and 1*054 volt. In these instances, and in the case 

 of the lower values obtained by other observers, doubtless the 

 "polarizations" produced by the passage of the tolerably 

 powerful currents employed were considerable. The highest 

 values were obtained with cells in which dilute sulphuric acid 

 was used; thus, H. F. Weber found that a perceptibly higher 

 value was obtained with such a cell than with one containing 

 zinc-sulphate solution, viz. 1*1317 and 1*1286 (mean = 1*1301), 

 as compared with 1*0954. That this should be the case is 

 predicable from the nature of the heat-evolutions taking- 

 place when zinc is dissolved in acid of various strengths. Let 

 an amount of heat, H 1? be evolved when a gramme- equivalent 

 of zinc oxide is dissolved in sulphuric acid of given strength, 

 S0 4 H 2 , m H 2 ; and let H 2 be the heat evolved on its solution 

 in acid of a different strength S0 4 H 2 , n H 2 O, n being less 

 than m. Let the solution S0 4 Zn, ™H 2 0, resulting in this 

 latter case, evolve li x of heat on the addition of (m—n) H 2 0, 

 so as to form the solution S0 4 Zn, m H 2 0; and let the heat 

 evolved on the addition of this quantity of water to S0 4 H 2 , 

 n H 2 0, so as to convert it into S0 4 H 2 , m H 2 0, be h 2 . Then, 

 if the zinc oxide were dissolved in the stronger acid, and 

 the zinc sulphate diluted, the heat-evolution would be H 2 + /ii ; 

 whilst if the acid were diluted first, and the zinc oxide were 

 then dissolved in it, the total heat evolved would be B^ + Zig. 

 Since of necessity the two amounts of heat, H 2 + h^ and 

 Hi + h 2 , must be equal, it results that H 3 -^ Hj — Aj + 7t 2 . 

 Now h 2 is a considerable positive quantity in all cases ; 

 whilst Thomsen's results on the heat evolved on solution of 

 salts in water {Deut.chem. Ges.Berichte, 1873, p. 710) indicate 

 that when the solution is accompanied by heat-absorption 

 (as is the case with zinc sulphate), a further heat-absorption 

 takes place on diluting a stronger solution of the salt with 

 water, so that h x has a negative value. Hence, on both 



